EuiComboBox: Persistent placeholder in input

[cjcenizal]

Per #567 (comment), we could improve the UX by keeping a persistent placeholder to prompt the user to use the combo box.

Examples from Uniform and react-select feel pretty intuitive without the persistent placeholder so I wonder if the component is better without it, since no placeholder would simplify it visually.

[snide]

I think you'll want one for the default empty in any case. Just to match against what every other one of our form components does.

I agree the persistent one isn't necessary, so feel free to close this if that's what you're referencing.

[cjcenizal]

Definitely. I'll implement the default placeholder in the original PR. Closing this, since it's just for the persistent placeholder.